The invention concerns a procedure for current monitoring in a power supply system and a current measuring installation suitable for this purpose.
A typical example for a power supply system of the type examined here is supply of the number of loads growing from year to year in motor vehicles due to increasing requirements made of comfort, safety and environmental compatibility. It proves to be advisable due to the correspondingly increased power consumption to connect all loads by a common power bus in ring or star connection to a power source of relatively high voltage of for instance 42V, which enables lower currents than the 12V batteries usual up to now and enabling correspondingly smaller line diameters. For instance, assuming overall consumption of up to 5 kW, then at 42V a maximum total current of 120 A results, which may still be easily handled in motor vehicles. The source of high voltage may of course be provided in addition to an on board network for lower voltages.
For exchange of control data and information between the also increasing number of different processors, actuators and sensors of motor vehicles, for instance for motor control and comfort electronics (window lift, automatic seat adjustment, airconditioning, etc.), largely the use of standard data bus systems (for instance the so-called CAN bus) has prevailed, which may be located in parallel to the power bus. For the required semiconductors, a power supply network with higher voltages like for instance 42V is also advantageous, because for different reasons the semiconductor surface for secure switching of a load is much smaller than in case of a 12V on board network.
With an increasing number of loads and rising overall power, on the other hand the danger of faults and especially of short circuits increases, whose consequences may form a catastrophe for the safety and function of the motor vehicle. The fire hazard is especially large. Of corresponding magnitude were the requirements for safety and functionality monitoring up to now, using fuses and current measuring elements typically individually associated to the individual loads.
The primary object of the invention is a method or respectively a current measuring device enabling in a simple manner at less expenditure as up to now, quick and as exact as possible current monitoring in power supply systems, for instance of a motor vehicle with numerous loads.
This task is solved by the characterizing features of the patent claims.
According to a first aspect of the invention, each load is not associated with its own current sensor, but several or even all loads are preceeded by a single sensor resistor in series with the power bus for permanent current monitoring. Current sensing is performed quickly and precisely and may be temporally exactly associated to switching a load on or off by means of the control bus. Because the respective actual current value is directly accessible to any existing processor or other control device by means of the control bus, the electronic control and monitoring system may determine without any problem if for the respective switching process normal current changes result or not. For instance, it is possible to determine the load of actuators from their current consumption and therefore faulty functionality or faults of the respective motors (window lifters, starter, ventilator, etc.) may be detected. Specifically permanent sensing of the total current in the vehicle may be of great importance for detection of faults. The current of a generator feeding the power bus may also be monitored.
Because many or all loads and possibly the generator may be monitored using a single current sensor resistor, it is possible to omit numerous current sensors and safety circuits required up to now, like for instance individual lamp current sensing, fuses, etc. Beyond this, totally new opportunities for safety and functionality tests are created. The control system may also generate switching signals for any control processes from evaluation of the measured differential current, for instance to switch off individual loads not urgently needed, like for instance the rear-window heater, if the total current is too high.
Location of the measuring resistor within or in series with the power bus according to the invention also in a simple manner enables current control to protect the battery feeding the bus and the connected loads, especially with the goal of extending the service life of the battery, By measuring the total current flowing on the bus, it is also possible to monitor and limit the charging and discharging current of the battery.
Which monitoring is in detail undertaken within a vehicle essentially is a question of the existing control system and the associated software. But the design of the current sensor could be same for all applications.
The constructional control expenditure may be reduced to a minimum by the fact that, according to the current semiconductor technology, it is possible to integrate a complete control unit for the control bus together with the circuits required for the measuring resistor like measuring amplifier, A/D converters, processor, memories, etc. in a single small semiconductor chip as application specific integrated circuit (ASIC) which may be produced cheaply.
Of decisive importance for desired quick and extremely precise current sensing are on the one hand the measuring precision of the measuring resistor used and on the other hand interference and noise free transmission of the measured voltage of the resistor to the control system. For this purpose, besides avoiding external interfering signals, it is especially necessary to be independent of temperature effects.
These problems are solved according to an important other aspect of the invention by means of a current measuring module for which the semiconductor chip associated to the resistor is located directly or via a heat-conducting circuit board or PCB on the metal surface of the resistor, which is preferably plate-shaped, the chip preferably being situated in the center of the resistor surface. Besides its small volume and simple, cost-efficient production, this current measuring module has the advantage of extremely short connections between the power connections of the measuring resistor and the associated semiconductor circuits with the consequence that no substantial interfering voltages may be induced. Beyond this, due to thermal coupling, optimal temperature compensation is possible. The resistor should preferably be a component with a plate-shaped resistor element consisting of a resistor alloy, at whose edges in known manner (EP 0 605 800 B1) plate-shaped connectors are welded. The resistor may be produced with a very small resistance value, for instance of the magnitude of 0.1-0.5 mOhms, which means correspondingly small dissipation power even in case of substantial currents, which in the case of vehicles may for a short time amount up to 500 A.
Current measuring modules of the design described here may be employed for sensing the total current of several or all loads of the system, or for individual current measurement be connected into the power circuit of individual loads or individual groups of (for instance 2) loads.